Conventionally, MRI has been used to produce images by exciting the nuclei of hydrogen atoms (present in water molecules) in the human body. MRI imaging with polarized noble gases can produce improved images of certain areas and regions of the body. Polarized Helium-3 (“3He”) and Xenon-129 (“129Xe”) have been found to be particularly suited for this purpose.
Hyperpolarizers are used to produce and accumulate polarized noble gases. Hyperpolarizes artificially enhance the polarization of certain noble gas nuclei (such as 129Xe or 3He) over the natural or equilibrium levels, i.e., the Boltzmann polarization. Such an increase is desirable because it enhances and increases the Magnetic Resonance Imaging (“MRI”) signal intensity, allowing physicians to obtain better images of the substance in the body. See U.S. Pat. No. 5,642,625 to Cates et al. and U.S. Pat. No. 5,545,396 to Albert et al., the contents of which are hereby incorporated herein by reference as if recited in full herein.
In order to produce the hyperpolarized gas, the noble gas is typically blended with optically pumped alkali metal vapors such as rubidium (“Rb”). These optically pumped metal vapors collide with the nuclei of the noble gas and hyperpolarize the noble gas through a phenomenon known as “spin-exchange.” The “optical pumping” of the alkali metal vapor is produced by irradiating the alkali-metal vapor with circularly polarized light at the wavelength of the first principal resonance for the alkali metal (e.g., 795 nm for Rb). Generally stated, the ground state atoms become excited, then subsequently decay back to the ground state. Under a modest magnetic field (10 Gauss), the cycling of atoms between the ground and excited states can yield nearly 100% polarization of the atoms in a few microseconds. This polarization is generally carried by the lone valence electron characteristics of the alkali metal. In the presence of non-zero nuclear spin noble gases, the alkali-metal vapor atoms can collide with the noble gas atoms in a manner in which the polarization of the valence electrons is transferred to the noble-gas nuclei through a mutual spin flip “spin-exchange.”
Conventionally, lasers have been used to optically pump the alkali metals. Various lasers emit light signals over various wavelength bands. In order to improve the optical pumping process for certain types of lasers (particularly those with broader bandwidth emissions), the absorption or resonance line width of the alkali metal can be made broader to more closely correspond with the particular laser emission bandwidth of the selected laser. This broadening can be achieved by pressure broadening, i.e., by using a buffer gas in the optical pumping chamber. Collisions of the alkali metal vapor with a buffer gas will lead to a broadening of the alkali's absorption bandwidth.
However, since large-scale continuous flow polarizers were first introduced, their performance levels, as measured by both polarization and production rate, have fallen far short of theoretical predictions (Driehuys '08, Mortuza '03, Norquay '13). The first continuous-flow 129Xe polarizer achieved just 2-5% polarization at production rates of 1 liter/hr, while using 140 W of laser light (Driehuys '96). This stood in stark contrast to predicted polarization levels of 60-80% and production rates of 2.5 L/hr.